Possible Evidence for Dark Energy

by Paul Gilster on July 30, 2008

If dark energy is accelerating the expansion of the universe, how can we identify its signature? Researchers at the University of Hawaii have been using microwaves to detect what they believe to be dark energy at work. If their work stands up, it will be a useful step for cosmology, but also a potential boon for those of us with interstellar travel in mind. We obviously want to understand a force that may one day have propulsion implications, and it’s possible that the universe is offering a set of useful clues. Here cosmology and propulsion science share a common interest.

Led by István Szapudi, the researchers zeroed in on galactic superclusters — the largest structures in the universe — and so-called ‘supervoids,’ vast areas with few galaxies in them. Remember the prefix ‘super’ here, for conventional galactic clusters are some ten times smaller and held together by gravity, while the Hawaii team believes galaxies in the supervoids and superclusters are more affected by dark energy than gravity. See the team’s Web site for more on the nature of supervoids and superclusters and its use of the Sloan Digital Sky Survey.

The work proceeded by imposing supervoid and supercluster information on a map of the Cosmic Microwave Background, the most distant light visible to us, stretched by the expansion of the universe into the part of the spectrum we associate with radio rather than light. As microwaves pass through them, the superclusters and supervoids have a decided effect. Says Szapudi:

“When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster. Later, as it leaves the supercluster, it should lose exactly the same amount of energy. But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster.”

The image above gives an idea of the result. Microwaves passing through a supercluster are somewhat stronger than those passing through a supervoid. The team believes we are seeing dark energy at work as it stretches supervoids and superclusters to cool or heat light.

Image: These two images from the team’s paper produce spots that are highly significant; taken together, the spots have only a 1-in-200,000 chance of occurring randomly. This is arguably the clearest detection of the ISW effect (see below) to date. It has been detected before at about the same statistical significance, but those detections involve a somewhat cumbersome combination of galaxies from various heterogeneous galaxy samples (the team used a single sample). Credit: István Szapudi/University of Hawaii.

By ‘ISW,’ the team refers to the Integrated Sachs-Wolfe effect, which is responsible for the heating or cooling of photons as they pass through the supercluster and supervoid areas — if interpreted correctly, this is a direct signal of dark energy. You can read a University of Hawaii news release here, while the paper to study first is Granett et al., “Dark Energy Detected with Supervoids and Superclusters,” a look at the investigation that will be reported in the Astrophysical Journal in shorter form.

re: “The problem with dark energy is that I have never seen any proposal to recreate and/or test for in in a laboratory. All of the other forms of energy can be created and tested for in a laboratory”

The only reason dark energy has that name is to do with the way that its (presumably gravitational) effect scales with the volume of the universe. It doesn’t look like any form of condensed matter, and that’s the best we can do to identify it.

If it really is a ‘pure’ form of energy such as tension in spacetime then the only way we’ll ever recreate it in a lab is by making a few universes of our own to poke.

Abstract: We highlight the unexpected impact of nucleosynthesis and other early universe constraints on the detectability of tracking quintessence dynamics at late times, showing that such dynamics may well be invisible until the unveiling of the Stage-IV dark energy experiments (DUNE, JDEM, LSST, SKA).

Given that such models are arguably the best-motivated alternatives to a cosmological constant these results may significantly impact future cosmological survey design and imply that dark energy may well be dynamical even if we do not detect any dynamics in the next decade.

Abstract: Due to quantum fluctuations, spacetime is foamy on small scales. The degree of foaminess is found to be consistent with the holographic principle.

One way to detect spacetime foam is to look for halos in the images of distant quasars. Applying the holographic foam model to cosmology we “predict” that the cosmic energy density takes on the critical value; and basing only on existing archived data on active galactic nuclei from the Hubble Space Telescope, we also “predict” the existence of dark energy which, we argue, is composed of an enormous number of inert “particles” of extremely long wavelength. We speculate that these “particles” obey infinite statistics.

Comments: 6 pages, LaTeX, talk given at the Fourth International Workshop on the Dark Side of the Universe in Cairo (June 1-5, 2008), to appear in the Proceedings

Astronomical instruments needed to answer crucial questions, such as
the search for Earth-like planets or the way the Universe expands,
have come a step closer with the first demonstration at the telescope
of a new calibration system for precise spectrographs. The method
uses a Nobel Prize-winning technology called a ‘laser frequency
comb’, and is published in this week’s issue of Science.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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